The ball mill was optimised based on the operational parameters involving the fractional ball filling (J), the fraction of critical speed ( c), powder filling (U), solids concentration, ball diameter (d) ball and feed size distributions, to achieve a given objective function which could be to Jul 03, 2012 Ball Mill Classification. Ball mills are also classified by the nature of the discharge. They may be simple trunnion over-flow mills, operated in open or closed circuit, or grate discharge (low-level discharge) mills. The latter type is fitted with discharge grates between the cylindrical mill body and the discharge trunnion. The pulp can flow ...